Abstract

The identification of factors that cause the patchy distribution of organisms in space and time within natural ecosystems is a central concern in ecology. In running waters, disturbance by bed-moving flows has been recognized to contribute to this patchiness, but the mechanisms behind this process are still poorly understood. Recent research has revealed that most bed-moving spates and floods cause a small-scale mosaic of stream bed patches of different substratum stabilities (sediment scour, sediment deposition, and stable patches). The aim of the present research was to investigate the separate and combined roles of this “local disturbance history” and microhabitat parameters in determining the small-scale distributions of benthic stream organisms (bacteria, algae and invertebrates).
The first three chapters of my thesis investigate bacteria, algae and invertebrates after natural spates and floods in two gravel-bed streams, the Schmiedlaine in Bavaria, Germany, and the Kye Burn in the South Island of New Zealand. The final chapter describes a manipulative experiment in the Schmiedlaine in which the stream bed was disturbed manually to obtain a better understanding of how disturbance history affects stream organisms.
Local bed movement patterns during several natural disturbances were determined using metal-link scour chains. The same chains were also used as reference points when selecting stream bed patches for manipulation during the experimental disturbance. Quantitative samples were collected several times from random sites before each disturbance and from scour, fill and stable patches after each disturbance. In addition to sampling bacteria, algae and invertebrates, we measured primary production under controlled conditions in both streams and bacterial production in the Kye Burn. For each sampling site, we determined several abiotic and biotic microhabitat parameters that are known to influence the distributions of stream biota, e.g. water depth, substratum composition, near-bed current velocity and standing stock of particulate organic matter (POM).
The results of the correlative studies suggest that local disturbance history plays an important role in determining the small-scale distributions of stream bacteria, algae and invertebrates. Disturbance history was related to distributions of all three organism groups both directly and indirectly (via effects on habitat parameters) and significant habitat parameter effects revealed previously undetected history effects in several cases, indicating a strong interaction between disturbance history and the better explored habitat parameters.
Bacterial microdistribution was related directly to disturbance history at least as often as to any of the other nine studied habitat parameters (except for the surface area of the sampled stones in the Schmiedlaine). The relationships of local disturbance history with algal distributions were not quite as strong. In the flood-prone Schmiedlaine, substratum characteristics and current velocity were related to algal distribution at least as often as disturbance history. In the relatively more stable Kye Burn, there were no direct influences of disturbance history on the algae. Substratum characteristics, water depth, current velocity and POM were the habitat parameters with the highest numbers of significant relationships. However, disturbance history also influenced algal distribution indirectly in both streams, and this indirect influence was particularly strong in the Kye Burn. Primary production was always similar across bed stability types in both streams. Averaged across both streams, invertebrate distribution was related only to near-bed current velocity more often than to disturbance history. In the Kye Burn, disturbance history and water depth were the habitat parameters that were related most often to invertebrate distributions. Further important parameters for invertebrates were substratum size and epilithic algal biomass.
The observed relationships of benthic densities to the investigated habitat parameters were largely in accordance with previous microhabitat studies for benthic algae and invertebrates. These results also show that benthic bacteria in streams appear to be influenced by similar habitat parameters as benthic algae and invertebrates.
The findings of the experimental disturbance in the Schmiedlaine generally supported those of the correlative research, lending more weight to the generality of the results. Local disturbance history influenced directly algal microdistribution in the stream bed most often, followed by near-bed current velocity, total invertebrate density and the surface area of the sampled stone. Primary production was also always similar between the three bed stability types. In contrast to the benthic algae and the correlative study on bacteria, bacterial microdistribution in the experiment was influenced directly solely by stone surface area. However, disturbance history had a strong indirect influence via habitat parameters on bacterial distribution. For invertebrate distribution, disturbance history was the parameter with the second-highest number of significant relationships after current velocity. Indirect effects of disturbance history on the stream organisms were similarly common as in the correlative studies and significant habitat parameter effects revealed previously undetected history effects in several cases, confirming the strong interaction between disturbance history and the better- explored habitat parameters.
The combined findings of the present research provide substantial evidence that local disturbance history is an important, and previously unrecognised factor contributing to the patchy distribution of stream organisms in space and time.